# Trigonometry Trigonometry (from Greek words 'trigōnon' for triangle, and 'metron' for measure) **is a branch of mathematics that studies relationships between side lengths and angles of triangles.** In trigonometry angles are usually expressed using [[Radian|radians]]. **Sine ($\sin$) and cosine ($\cos$) are the two basic [[Trigonometric Functions]]. All other functions are derived from those two**, although one could argue that only one is enough, based on the Pythagorean Identity: ![[Trigonometric Functions#^m8vwo0]] Viewing both of them as basic helps though when defining their derivatives. See [[Derivatives of Sine and Cosine]] for more. ## Theorems - [[The Law of Sines]] - [[The Law of Cosines]] - [[Euler's Formula]]